Effects of Cooling Rate and Physical Aging on the Gas Transport Properties in Polycarbonate

The gas transport properties of bisphenol A polycarbonate films were examined as a function of the cooling rate during film processing and subsequent physical aging. Interpretation of the permeation properties using results from dynamic mechanical analysis and density measurements indicated that the diffusion coefficient of small gas molecules in glassy polycarbonate is influenced more by the local chain dynamics rather than by the overall free volume content. A one-dimensional lattice model was developed to help probe the effects of the distribution of energy barriers associated with polymer motion on the transport properties. Fast cooling rates generated highly restricted conformations, which hindered local motions leading to increases in the measured activation energy of diffusion. These results correlated well with a lattice model having a broad distribution of energy barriers. Annealing reduced the measured diffusion coefficients, which was shown to be consistent with narrowing of the energy barrier distribution, as well as an increase in the mean barrier energy. Interestingly, the decrease in the diffusion coefficient with aging was found to occur much more slowly in fast-cooled samples, despite the higher initial free volume content. By contrast, properties such as density or isothermal dynamic mechanical properties were more consistent with the free volume model.